Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive at lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
Positive working photoresists when they are exposed image-wise to radiation have those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the formation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Negative working photoresists when they are exposed image-wise to radiation, have those areas of the photoresist composition exposed to the radiation become insoluble to the developer solution while those areas not exposed remain relatively soluble to the developer solution. Thus, treatment of a non-exposed negative-working photoresist with the developer causes removal of the unexposed areas of the coating and the formation of a negative image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many leading edge manufacturing applications today, photoresist resolution on the order of less than 100 nm is necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such. demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm, are often used where subhalfmicron geometries are required. Particularly preferred are photoresists comprising non-aromatic polymers, a photoacid generator, optionally a dissolution inhibitor, and solvent.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. To date, there are three major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these use lasers that emit radiation at 248 nm, 193 nm and 157 nm. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers, such as those described in U.S. Pat. Nos. 4,491,628 and 5,350,660. On the other hand, photoresists for exposure below 200 nm require non-aromatic polymers since aromatics are opaque at this wavelength. U.S. Pat. No. 5,843,624 and GB 2320718 disclose photoresists useful for 193 nm exposure. Generally, polymers containing alicyclic hydrocarbons are used for photoresists for exposure below 200 nm. Alicyclic hydrocarbons are incorporated into the polymer for many reasons, primarily since they have relatively high carbon hydrogen to ratios which improve etch resistance, they also provide transparency at low wavelengths and they have relatively high glass transition temperatures. U.S. Pat. No. 5,843,624 discloses polymers for photoresist that are obtained by free radical polymerization of maleic anhydride and unsaturated cyclic monomers, but the presence of maleic anhydride makes these polymers insufficiently transparent at 157 nm.
Two basic classes of photoresists sensitive at 157 nm, and based on fluorinated polymers with pendant fluoroalcohol groups, are known to be substantially transparent at that wavelength. One class of 157 nm fluoroalcohol photoresists is derived from polymers containing groups such as fluorinated-norbornenes, and are homopolymerized or copolymerized with other transparent monomers such as tetrafluoroethylene (Hoang V. Tran et al Macromolecules 35, 6539, 2002, WO 00/67072, and WO 00/17712) using either metal catalyzed or radical polymerization. Generally, these materials give higher absorbencies but have good plasma etch resistance due to their high alicyclic content. More recently, a class of 157 nm fluoroalcohol polymers was described in which the polymer backbone is derived from the cyclopolymerization of an asymmetrical diene such as 1,1,2,3,3-pentafluoro-4-trifluoromethyl-4-hydroxy-1,6-heptadiene (Shun-ichi Kodama et al Advances in Resist Technology and Processing XIX, Proceedings of SPIE Vol. 4690 p76 2002; WO 02/065212) or copolymerization of a fluorodiene with an olefin (WO 01/98834-A1). These materials give acceptable absorbance at 157 nm, but due to their lower alicyclic content as compared to the fluoro-norbornene polymer, have lower plasma etch resistance. These two classes of polymers can often be blended to provide a balance between the high etch resistance of the first polymer type and the high transparency at 157 nm of the second polymer type.
In order to further improve the resolution and depth of focus of photoresists, immersion lithography is a technique that has recently been used to extend the resolution limits of deep uv lithography imaging. In the traditional process of dry lithography imaging, air or some other low refractive index gas, lies between the lens and the wafer plane. This abrupt change in refractive index causes rays at the edge of the lens to undergo total internal reflection and not propagate to the wafer (FIG. 1). In immersion lithography a fluid is present between the objective lens and the wafer to enable higher orders of light to participate in image formation at the wafer plane. In this manner the effective numerical aperture of the optical lens (NA) can be increased to greater than 1, where NAwet=nisinθ, where NAwet is the numerical aperture with immersion lithography, ni is refractive index of liquid of immersion and sinθ is the angular aperture of the lens. Increasing the refractive index of the medium between the lens and the photoresist allows for greater resolution power and depth of focus. This in turn gives rise to greater process latitudes in the manufacturing of IC devices. The process of immersion lithography is described in ‘Immersion liquids for lithography in deep ultraviolet’ Switkes et al. Vol. 5040, pages 690-699, Proceedings of SPIE, and incorporated herein by reference.
For 193 nm and 248 nm and higher wavelengths immersion lithography, water is of sufficient inherent transparency so that it can be used as the immersion fluid. Alternatively, if a higher NA is desired, water's refractive index can be increased by doping with UV transparent solutes. However, for 157 nm lithography, water's high absorbance makes it unsuitable as an immersion fluid. Currently certain oligomeric fluorinated ether solvents have been used as suitable immersion fluids.
One important concern in immersion lithography is the extraction of components from the photoresist film into the immersion fluid. These components may either be ones present in the film prior to exposure (e.g. base additives, photoacid generators, solvent, dissolution inhibitors, plasticizers, leveling agents,) or present in the film during or shortly after exposures (e.g. photoacid, photoacid generator, photofragments, scission fragments from the polymer or the other additives, salt of the photoacid and base additive.) The extraction of these materials is of concern for two reasons: firstly, it may affect resist performance deleteriously, and the second is the deposition of UV absorbing films on the objective lens in contact with the immersion fluid due to the photoreaction of extracted components in the immersion fluid.
Thus there is a need for a barrier coat having good optical transparency at the exposure wavelength, which can be spun onto the photoresist from a solvent system which will not redissolve the photoresist, and where the barrier coating layer is also insoluble in the immersion liquid, but can be removed easily during the normal aqueous base development step.
It is also known that chemically amplified photoresists, especially those based on the catalytic deprotection of an acid labile group, are particularly sensitive to amine contamination from the environment. The presence of amines can poison the acid generated during the photolytic process and neutralize the acid necessary for the deprotection of the polymer. This phenomenon is known and described in U.S. Pat. No. 5,750,312, where an acidic barrier coat is coated on top of the photoresist. Protection of the photoresist is particularly desirable for instances where a chemically amplified photoresist is exposed in air or other gases. U.S. Pat. No. 5,750,312 particularly describes acid polymers based on carboxylic acids, such as, poly(methacrylate-co-methacrylic acid) and poly(benzyl methacrylate-co-methacrylic acid) coated over a photoresist sensitive at 248 nm. Such top coats cannot be used for photoresists sensitive at 193 nm and at 157 nm, since the top coats described in U.S. Pat. No. 5,750,312 have insufficient transparency at 193 nm, and especially 157 nm. Thus there is a need for new transparent polymers that can act as effective barrier top coats for 193 nm and 157 nm exposure wavelengths.
The inventors of this application have found that, surprisingly, a barrier coating composition comprising certain polymers and an alkyl alcohol solvent can be employed as effective barrier against removal of photoresist components or photoresist photoproduct during the imaging process using immersion lithography. Additionally, the inventors have found that polymers comprising an acidic fluoralcohol group may be used as top barrier coats for the prevention of amine contamination of the photoresist, when exposure is undertaken in air or other gases.